CARBONATE PRECIPITATION FROM METEORIC-HYDROTHERMAL FLUIDS AT A DETACHMENT FAULT, SOUTHERN WHITE PINE RANGE, NEVADA: EVIDENCE FROM CLUMPED ISOTOPES

The Currant Gap Detachment Fault (CGD) was documented to host widespread silica replacement of carbonate by meteoric-hydrothermal fluids (Francis et al., 2021; Holk et al., in press). Phyllosilicates that occur as inclusions in replacement silica and carbonate constrain this silicification event to have occurred between 150°C and 250°C. Petrographic analysis indicates that pervasive silica replacment of carbonate by fluids migrating through primary pore spaces and fractures occurred. Stable isotope data from altered intrusive sills and some silicified rocks document a meteoric-hydrothermal source (δ²H ~ –140, δ¹⁸O ~ –18) for fault-related fluids in or proximal to the CGD. Variable silica δ¹⁸O values from the CGD indicate that these fluids underwent variable degrees of ¹⁸O/¹⁶O exchange as they circulated through the crust prior to silicification.

Carbonate veins and breccia fillings are common within the CGD, in the lower plate Cambrian Pole Canyon Limestone, and in attenuated upper plate Paleozoic units dominated by carbonate. Gasbench d¹⁸O values of vein/breccia calcite indicate variable degrees of ¹⁸O/¹⁶O exchange between meteoric-hydrothermal fluids and rocks as they circulated through the system, similar to the silicates. Both δ¹⁸O and δ¹³C values obtained by the clumped isotope method match those acquired using the GasBench. Temperatures from carbonate clumped isotope geothermometry of selected vein samples range from 95°C to 200°C with the highest temperatures recorded within the CGD. These temperatures are much lower than those estimated from ¹³C/¹²C and ¹⁸O/¹⁶O exchange modeling of published data from conventional analysis (up to 400°C; Holk et al., in press). Except for one high δ¹⁸O vein collected within a meter of a lower plate intrusion (T = 170°C, δ¹⁸O_H2O = 5.3), all carbonate veins have a positive correlation between temperature and fluid δ¹⁸O values, which range from –16.7 to –5.9. This trend is consistent with the higher temperature fluids having been transported a longer distance through the crust prior to carbonate precipitation, as opposed to a shorter fluid pathway for the lower temperature veins/breccias. Solubility relationships between carbonate and H₂O suggest that precipitation occurred as meteoric-hydrothermal fluids migrated downward into the crust. Clumped isotopes from one host rock sample indicate T = 89°C and a fluid δ¹⁸O = –5.0.